The objective is to develop a wireless, wearable device for non-invasive assessment of continuous, long-term (multi-day) beat-by-beat systolic and diastolic blood pressure (BP) in humans without use of cuff inflation or other arousing or activity-interfering stimulation. The proposed non-invasive continuous BP (CBP) methodology addresses an unmet need for determination of extreme transient BP changes during obstructive sleep apnea, following ictal periods with potential hypotension leading to sudden death in epilepsy, momentary pressure changes in chronic hypertension and hypotensive patients, and numerous other cardiovascular and respiratory conditions where BP changes accompany disease progression. The device will be useful in patients who are likely to display sudden changes in blood pressure (e.g. vascular surgery), in whom close control of blood pressure is required (e.g. head-injured patients), or in patients receiving drugs to maintain blood pressure. Collectively, the health care needs of over half of the US population at some point in their life could benefit from the device. CBP monitoring provides numerous advantages. The device is very low cost, allowing ready replacement of ambulatory cuff-based 24hr devices. Instead of the intermittent, spotty collection and arousal induction of automated BP cuff monitors, the device offers continuous (i.e., beat-by- beat) data, an obvious advantage in BP assessment during sleep or surgery. The CBP technique allows accurate blood pressure readings at very low pressures, for example, during shock, a critical aspect in emergency transport or care circumstances, or in surgery. The device is wireless, avoiding multiple electrode placements, requires only a single thoracic and wrist placed sensor, and thus significantly improves patient comfort, especially in long-term blood pressure monitoring, e.g. ICU or chronically-ill home-based patients; moreover, repeated cuff inflations are avoided. The sensor design allows estimates of intravascular volume status from the wave shape of the arterial pressure data, providing valuable insights into cardiovascular dynamics and BP measures, which are impervious to distortions commonly induced by pulse transit time BP-inference devices due to heart rate variations. In this Phase I proposal, we will continue to develop the prototype, and provide initial validation in patients with intra-arterial line BP data to CBP undergoing cardiovascular procedures, and in a set of three levels of ambulatory hypertensive patients. The initial clinical evidence will provide support for a later Phase II proposal in a full clinical trial.